This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
Congestive heart failure (CHF) is one of the leading causes of admission to the hospital [1]. Studies have shown that patients with dilated hearts have a reduction in the frequency of hospital admission and prolongation of life with the implantation of bi-ventricular pacemakers and automatic implantable cardiac defibrillators [AICDs, 2-6]. Recently, “piggybacking” technology onto AICDs and bi-ventricular pacemakers for sensing the progression of impending CHF to reduce the number and length of stay of hospital admissions for CHF has been proposed [7-18]. There are two clinically tested “piggybacked” heart failure warning systems placed on bi-ventricular pacemakers and AICDs to reduce hospital admissions. First, Chronicle® measures right heart pressures in an attempt to monitor increases that are indicative of heart failure [11-13]. Second, Optivol® and CorVue® use lung conductance measurements as an indication of pulmonary edema [8-10, 15]. However, both are downstream measures of the earliest indicator of impending heart failure—left ventricular (LV) preload or left ventricular end-diastolic volume (LVEDV).
Conductance measurements have been available as an invasive tool to detect instantaneous LV volume since 1981 [25, 26]. Conductance tetrapolar electrodes are usually placed on a lead located within the heart chamber to determine instantaneous volume (FIG. 2). Conductance systems generate an electric field (22) using a current source and volume is determined from the returning voltage signal. Prior art shows how to separate the blood and muscle components from the combined voltage signal to determine LV preload from previously implanted AICD and bi-ventricular pacemakers.
Significant improvement in patient care could be achieved by adding the admittance apparatus [19-24] to pacemakers and AICDs, using currently deployed bi-ventricular and AICD leads, to electrically detect either true LV preload, or an increase in LV preload from baseline. Bi-ventricular and the RV AICD leads are already located in the ideal locations—the lateral LV epicardium and the right ventricular (RV) septum (FIGS. 1a, 1b). Since blood has 5-fold lower resistivity than myocardium, the preferential path (22) for a substantial fraction of the current flow will be the LV blood volume. This low-power admittance apparatus can be “piggy-backed” onto implanted AICD and bi-ventricular pacemakers to serve as an early warning system for impending heart failure. Piggy-backed means one can take an existing pacemaker design and add this apparatus to it, without major redesign of the pacemaker itself. This means the admittance circuits need not be included in one of the internal pacemaker chips; rather, it could be added to the system without redesigning the pacemaker circuits themselves. This is particularly true because much of this invention involves a change in software requiring only modest changes in hardware. particular, the apparatus uses the same leads, the same communication channel, and the same power source as the pacemaker. The apparatus can be triggered by the pacemaker or it can run untriggered (i.e., it runs periodically). The output of the apparatus will be a true/false warning signal, or a quantitative measure of heart volume. In this configuration, the apparatus does not alter how the pacemaker operates.
On the other hand, if one wished to design a new pacemaker, this apparatus can also be used to dynamically adjust parameters in the pacemaker itself to maximize heart pumping efficiency. Furthermore, the volume information could be used to improve the effectiveness of ventricular tachycardia detection in an automatic defibrillator.
A version of this apparatus can be implanted in animals (including, but not limited to mice, rats, dogs, and pigs), which includes a pressure channel and a wireless link (FIGS. 5 and 14). The lead is placed in a ventricle (FIG. 2), and the experiment duration can vary from 1 day to 6 months. The duration of the experiment is limited by the animal survival and the storage capacity of the battery, and not the operation of the apparatus. The apparatus measures heart muscle function (left ventricular pressure-volume relationships) in these animals, and can be used for new drug discovery. These animals must be un-tethered and freely roaming so that the transmitted signals are physiologic. The apparatus transmits pressure-volume data (52), and a computer-based receiver collects, displays, and stores the data.
There are two papers describing a technique to detect heart failure [27-28] that may seem similar to the approach herein. In this technique, the current source and sink electrodes are both in the RV, and the sensing electrodes on the LV free wall. This means the electrical field will be confined to the RV. So even though they have sensing electrodes on the LV wall, their signal will have a very weak dependence on the left heart volume, detecting only fringing fields. Their type of measurement is very noise prone, and this problem worsens as the heart enlarges because the septum blocks much of the field from reaching the LV free wall. It is believed that the present invention is superior because 1) the majority of the sensing field goes across the blood pool in the left ventricle due to the relative conductivity of blood being high, and source and sink electrodes being placed on opposite sides of the LV, and 2) because the heart muscle is removed as an artifact of the measurement using admittance. Therefore, it is believed there are no available or proposed technologies that can perform chronic volume measurements of the left ventricle, including LVEDV, LVESV, and LVSV.